The cellular immune system of vertebrates consists of both innate (e.g., NK cell), and adaptive (e.g., T-cell) components. In both of these, immune function is achieved primarily through interactions with MHC molecules. The phenomenon of all recognition, manifested as a MHC-based histocompatibility reaction, is thought to be a by-product of this interaction coupled to the highly polymorphic nature of the MHC. It is dogma that MHC polymorphism is a result of the evolutionary battle between organisms and rapidly multiplying microbes. In contrast, we are studying an ancestral vertebrate that uses allorecognition to prevent the natural transplantation of predatory stem cell lineages. This organism is the colonial protochordate, Bottrylus schlosseri. When two colonies of B. schlosseri come into contact, extracorporeal blood vessels reach out and either fuse together, or reject each other in an active, blood-based immunity. This reaction is controlled by a single, highly polymorphic locus called the Fu/HC. Two individuals will fuse if they share one or both alleles at this locus, and reject if they share neither. Upon fusion of histocompatible colonies, bloodborne totipotent somatic and germline progenitors transfer between them, and have the ability to parasitize both the soma and germline of the other individual. A comparable vertebrate condition is fetuses sharing a common blood circulation, such as in freemartin cattle. The goals of this proposal are to isolate the Fu/HC using a map based cloning strategy, characterize the expression, structure and function of Fu/HC gene products, and to analyze their role in histocompatibility. Given the close phylogenetic relationship between the protochordates and vertebrates, Fu/HC-based allorecognition may represent the ancestral MHC, or a completely novel innate allorecognition system.